Hello ferdywallinx,
Depth filtering may take a long time due to excessive overlap. Usually the process is slowed significantly when there are overview photos - images that overlap with tens and even hundreds of other images. For example, for close range objects it means that there are mostly close-up images but some photos include the complete object in the frame. Or if speaking of aerial survey, the images are taken from several heights and the ones taken from the higher altitude have lower GSD, resulting in the overlap with considerable amount of lower altitude photos.